In solar thermal energy systems, the rays of the sun are concentrated to heat a fluid to high temperature (generally in the range of 300-550° C.). Typically, the heated fluid is piped from the solar concentrator to drive a turbine in order to generate electricity.
Like other types of solar energy systems, however, solar thermal energy systems suffer from the inherent problem that they collect energy only while the sun is shining at a sufficiently high elevation, i.e., substantially less than 50% of the time on average. Furthermore, the hours of energy collection generally do not overlap with the hours of peak electrical energy consumption. Various solutions have therefore been proposed and put into practice to store collected thermal energy for use in driving turbines during off-hours. Most of the interest in this regard has focused on materials that have high heat capacity and are also good thermal conductors, such as molten salts.
Other sorts of materials for heat storage have also been proposed and tested. For example, Robert H. Turner describes experimental evaluation of various materials for such purposes in “High Temperature Thermal Energy Storage in Steel and Sand,” JPL Publication 80-35 (Jet Propulsion Laboratory, Pasadena, Calif., 1979). This study evaluated hollow steel ingots, pipes embedded in concrete, and pipes buried in sand. It concluded that for large-scale thermal storage units, the sand-pipe approach offers attractive possibilities.